Conventionally, as a low-profile display, liquid crystal displays using liquid crystal have become widespread. On the other hand, an organic electroluminescent element (OLED) is a self-emissive element, and is capable of high contrast display, has fast response speed, and also has the advantage that since a backlight is not required reduced power consumption can be expected. For this reason OLED display are becoming widespread.
Here, among these types of display, the mainstream type is an active matrix type display having a driving transistor provided for each line, for controlling pixel display using the driving transistors. In the case of liquid crystals, it is possible for a driving transistor to control an applied voltage to the liquid crystal, but in the case of OLED, current flowing in the OLED must be controlled with the driving transistor.
Accordingly, in an OLED display, there are variations in output current between driving transistors, and if they are used as they are it is detrimental to display quality. On the other hand, the driving transistor of each line is formed using a silicon layer formed on a comparatively large glass substrate, which shows that it is difficult to make variations in the characteristics of the driving transistor, and in particular variations in the threshold voltage at which current starts to flow to the diving transistor, small. Therefore, in order to achieve improved display quality, there have been various proposals to correct the threshold voltage of the transistor used for driving and suppress variations in drive current (See U.S. Pat. No. 7,057,588, U.S. Patent Application Publication No. 2005-0206540 and JP 2006-259714).
However, in the above described related art U.S. Pat. No. 7,057,588, U.S. Patent Application Publication No. 2005-0206540 and JP 2006-259714, there are the following problems.
For example, with U.S. Patent Application Publication No. 2005-0206540, at the time of the signal voltage write process, the sampling transistor is made conductive to sample the signal voltage. At this time, the driving transistor is placed in an ON state because the threshold voltage has been exceeded. Accordingly, when writing the signal voltage it is easy for the threshold voltage held in a capacitance to drain away. In particular, as the sampling time for the signal voltage becomes longer and the signal voltage becomes larger, this reduction becomes more prominent. In order to prevent the drawback of this type of threshold voltage draining, a large capacitance is required, which increases the surface area of a constricted element, and it is also easy for the incidence of defects to rise.
With U.S. Pat. No. 7,057,588, it is necessary to provide three or four scanning lines for threshold voltage correction and for signal voltage sampling. Accordingly the structure is complicated and it is easy for incidence of defects to rise. Also, with JP 2006-259714, since drive current is affected by voltage variation of the OLED, there is a problem that degree of freedom for stable operation is narrowed.